doxygen: rework image support.

doxygen: rework image support.

 * The text after is not an alt text, but rather a figure caption. So
   render a figure in that case.
 * That means it's also not an error if the caption is not present. So
   don't warn about it.
 * Doxygen, if it wouldn't be so broken, would copy the images to the
   XML output directory as well. So there's no need to juggle with
   IMAGE_PATH. I love the smell of burning code.